Nitriding and Nitrocarburizing are well known diffusion processes for case hardening irons and steels so as to produce a high surface hardness, increase wear and heat resistance, and improve fatigue life and corrosion resistance.
In conventional gaseous nitrocarburizing of irons and steels, a gaseous atmosphere is employed to provide active nitrogen and carbon simultaneously to iron/steel components to form the desired epsilon phase on the surface. The most commonly used source for active nitrogen is ammonia and for active at least one of the carburizing gases such as natural gas, propane, carbon an endothermic or exothermic gas reaction produces carbon insitu. One typical atmosphere contains equal amount of ammonia and an endothermic gas and another costs of 35% ammonia and 65% refined exothermic gas which may be enriched with a hydrocarbon gas. Most gaseous nitrocarburizing processes are performed at about 565.degree. C., a temperature just below the austenite temperature range for the Fe-N-C system. The treatment times generally range from 1 to 5 h. However the treatment temperature cannot be raised as ammonia gas used for equilibration decomposes rapidly with increase of temperature, before reaching the surface of the components to be treated. Therefore, gaseous nitrocarburization cannot generally be applied at 700.degree. C.
Prior art processes include Nitemper.RTM. and Triniding.RTM. (Ipsen & Surface Combustion Inc. USA) 1961; Nitrotec (Lucas Electrical Co. UK) 1982; QPQ (Degussa, Germany) 1973; LT (Wuhan Inst. of Materials Protection, China) 1984 and FON (Tianjin Inst. of Textile Science and Technology, China) 1987. Attention is also directed to Published European Patent application 0323511, filed 16 Jun. 1987, laid open 12 Jul. 1989, Gorodetsky et al, which describes a low temperature process for nitrocarburizing using an organic polyamide reagent. A comparison of the products of these processes is presented in Table I below. Most processes are carried out at a temperature below 700.degree. C. Obviously Gorodestky's process is easier and simpler than the above nitrocarburization processes, and a diffusion zone of 60-90 3/4m depth may be formed in about 2-4h for alloy tool steels, but the nitrocarburizing process is still carried out at a low temperature range.
There is, therefore, a need for a nitrocarburizing process which is operable at a higher temperature in order to achieve maximum properties of the treated metal.
Table I ______________________________________ Com- pound Penetrated Time layer Diffusion Process element T(.degree.C.) (h) (.mu.m) zone(.mu.m) ______________________________________ Nitemper N + C 570 3 18 Triniding N + C 570 4 12 Nitrotec N + C + O 550-740 3 25 QPQ N + C + O 570 2 18-22 550 LT N + C + S 570 3 18-25 600 FON N + C + O 570 1 28 540 520-540 0.3-0.5 30-60* Goro- N + C 480-680 2-4 60-90* detsky ______________________________________ *high alloy steels